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Wed, 23 Sep 2026 22:10:15 -0700 (PDT) From: Mikhail Gavrilov To: Dave Hansen Cc: Dave Hansen , Andy Lutomirski , Peter Zijlstra , Thomas Gleixner , Ingo Molnar , Borislav Petkov , x86@kernel.org, "H . Peter Anvin" , Mike Rapoport , Lorenzo Stoakes , Pedro Falcato , Toshi Kani , linux-mm@kvack.org, regressions@lists.linux.dev, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] x86/mm: Drop the page allocation from pud_free_pmd_page() Date: Thu, 24 Sep 2026 10:10:11 +0500 Message-ID: <20260924051011.21237-1-mikhail.v.gavrilov@gmail.com> X-Mailer: git-send-email 2.55.0 In-Reply-To: References: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 8EAAE4000A X-Stat-Signature: auc7g4whomiha76uttyfudayogxzghcu X-HE-Tag: 1790226617-883002 X-HE-Meta: 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 sNS/f+Dp Pe4CiCopPVbCrrLSwkH7wFxh5ag89HC8eHPSuXVIlBIKcYhAjePI0hmMz8i4V+0IUxKmLkfIwGEXEJnOoDK5tvif0Y4ml6SLsck1vE+z3/N69BaRGQ6iAlf9uvSQm3yiMwcc+JO1/dZxudV1urBZDSt/JtV3f6pKjVaNapfdNgiT8+6z1s7w2lMV+z36DPLMzUSlmqjrC0i3pDgd/gxOjNR0akGdQCHpR6JhePZRkf9Pxu8kLuynK1X7slm260+J4NEh4W8e6GWIJplQ21RTZB4nVDo/0LAbtUnkyA+5/Jb5kG6sOHttLUeKIwboFvvuOnYo3e/Z+3X2rd8vb4HrEdHHnbD2PFfEX7PE7tkEmyg3WivehPyrPoteW+7JxzvY390Y5g81koZ1GfUnRijmQQEdwx44dESKgJjyc3R0SyKCmlAHUjrJtWwig5sTGHJWBBEezXLLEB9jJibeyB1uNpQn58gMVIDV0AuQKb4KphKHa9DNELFbq74mBK2g7bymz6ZfwVrUr8Wwmqc0fIT6Tb4L2f6NP1+KfvmxbMIERwipyWUXBNe9rYTAk5+MhQoQYNK+2z14bZeIlJmPKUbCMEIwgBRB+WnElQqn/OKFlsYsXWzM= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 9/23/26 16:07, Dave Hansen wrote: > There are a few things we need to figure out like having > pmd_free_pte_page() return 'int' when it doesn't ever fail, the > comments, and the reverse Christmas tree. One data point for the return value: the x86-PAE pmd_free_pte_page() does fail. It returns pmd_none(*pmd), and vmap_try_huge_pmd() relies on that to keep ioremap() from replacing an existing PTE table there (the comment above it is about the sync'd PMD entries). Huge PMD vmap is enabled on PAE whenever the CPU has PSE. pud_free_pmd_page() has no such case: the PUD level is 64-bit only on x86, and with v2 applied the x86-64 version, like the arm64, riscv and powerpc ones, always returns 1. > I'll dig into it a bit tomorrow and see what I come up with. Thanks. This box reaches pud_free_pmd_page() on every boot I have checked, so I can run whatever you come up with through the same boot-time kprobe and reproducer check.